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somatic nervous system has motor neurons that stimulate skeletal (voluntary) muscle. Your autonomic nervous system has motor neurons that stimulate smooth (involuntary) and heart muscle. Your autonomic nervous system is subdivided into the antagonistic sympathetic nervous system and parasympathetic nervous system. Sympathetic stimulation results in responses that help your body deal with stressful events including dilation of your pupils, release of glucose from your liver, dilation of bronchi, inhibition of digestive functions, acceleration of heart rate, secretion of adrenalin from your adrenal glands, acceleration of breathing rate, and inhibition of secretion of your tear glands. Parasympathetic stimulation calms your body following sympathetic stimulation by restoring digestive processes (salivation, peristalsis, enzyme secretion), returning pupils to normal pupil size, stimulating tear glands, and restoring normal bladder contractions. Your spinal cord, protected by membranes called meninges and your spinal column of bony vertebrae, starts at the base of your back and extends upward to the base of your skull where it joins your brain. The cord is composed mainly of interneurons and glial cells, which are all bathed by cerebrospinal fluid produced by your glial cells.

The Brain

Your brain, which has the consistency of soft-serve yogurt, is covered by protective membranes called meninges, and is housed in your skull. The evolutionary approach describes the brain’s evolution from more primitive organisms, reasoning that new types of behavior developed as each new layer of the brain evolved. According to one evolutionary model (triune brain), the human brain has three major divisions, overlapping layers with the most recent neural systems nearest the front and top. The reptilian brain, which maintains homeostasis and instinctive behaviors, roughly corresponds to the brainstem, which includes the medulla, pons, and cerebellum. Developmental psychologists call the brainstem the hindbrain. The old mammalian brain roughly corresponds to the limbic system that includes the septum, hippocampus, amygdala, cingulate cortex, hypothalamus; the thalamus, which are all important in controlling emotional behavior, some aspects of memory, and vision. The new mammalian brain or neocortex, synonymous with the cerebral cortex, accounts for about 80% of brain volume and is associated with the higher functions of judgment, decision making, abstract thought, foresight, hindsight and insight, language and computing, as well as sensation and perception. Developmental psychologists call the structures of the “mammalian brains” the forebrain. The surface of your cortex has peaks called gyri and valleys called sulci, which form convolutions that increase the surface area of your cortex. Deeper valleys are called fissures. The last evolutionary development of the brain is the localization of functions on different sides of your brain.

Localization and Lateralization of the Brain’s Function

Although multiple representations of information can be located within different areas of your brain, specific regions of your brain seem most critical in handling particular functions. This localization of structure and function has been identified for numerous regions (see Figure 7.1). Association areas are regions of the cerebral cortex that do not have specific sensory or motor functions, but are involved in higher mental functions, such as thinking, planning, remembering, and communicating. In general, crossing over of nerves sending information from one side of your body to the other side of your brain results in contralaterality, control of one side of your body by the other side of your brain.

Figure 7.1 Major structures of the brain in medial view.

Just as a map or globe can be divided into hemispheres and continents, your cerebral cortex can be divided into eight lobes, four in the left cerebral hemisphere and four in the right cerebral hemisphere (see Figure 7.2). Go to www.g2conline.org and click on 3-D Brain for a more detailed view